1. Field of the Invention
This invention relates to a liquid crystal display device provided with a plurality of insulation layers and a method of manufacturing the same.
2. Description of the Related Art
A conventional liquid crystal display device to be used for a television set or other visual image display applications normally comprises a pair of substrates arranged to oppose each other with a liquid crystal layer interposed therebetween. A plurality of striped display electrodes are formed on the opposing surface of each substrate, and the extending direction of the striped electrodes on one substrate is substantially perpendicular to that of the striped electrodes on the other substrate. A plurality of areas of the liquid crystal layer, which correspond to the opposing portions of one and the other striped electrodes, are used as a plurality of imaging elements, which are controlled by one and the other striped electrodes so as to display an image on the display device.
At least one of the two groups of the striped display electrodes is formed on an insulation film formed on the opposing surface of the substrate corresponding thereto. In a conventional color liquid crystal display device, for instance, a color filter made of thin film of an insulation material is formed on the opposing surface of one substrate, and one group of the striped display electrodes are formed on the color filter. Another type of the color liquid crystal display device, in which a plurality of striped electrodes are firstly formed on the opposing surface of one substrate and then a color filter is formed on the electrodes, is also known. In this color display device, a driving voltage applied to the striped electrodes on the opposing surface of the above described one substrate is dropped until the voltage reaches at the liquid crystal layer though the color filter. Therefore, in the latter color liquid crystal display device, the driving voltage to be applied to the electrodes must be higher than that of the former color liquid crystal display device, as much as the drops of the voltage in the color filter.
FIG. 1 shows a conventional liquid crystal display device, in which a color filter is formed on the opposing surface of one substrate and striped electrodes are formed on the color filter; In the drawing, reference numbers 1 and 2 respectively denote the upper and lower ones of a pair of transparent substrates made of glass. On the opposing surface of one of the substrates 1 and 2 (the lower one in FIG. 1), a plurality of striped red color filters 3R, a plurality of striped green color filters 3G, and a plurality of striped blue color filters 3B are formed to be arranged alternately and in parallel with each other. These color filters 3R, 3G and 3B are covered by a protective film 4 made of transparent resin such as polyimide and formed on the opposing surface of the substrate 2, and a plurality of display electrodes (transparent electrodes made of indium-tin-oxide (ITO), for example) 5 are formed to be striped on the protective film 4. It should be noted that the protective film 4 is not formed on the peripheral area of the opposing surface of the substrate 2.
Terminals 5a of the striped electrodes 5 are arranged in the peripheral area of the opposing surface of the substrate 2 that is not covered with the protective film 4, because the ITO electrodes as a material for the electrodes does not have a good adherence to the protective film 4 made of polyimide resin for example. Therefore, if the terminals 5a of the electrodes 5 are formed on the protective film 4, the terminals 5 will be easily separated from the protective film 4. On the contrary, if the terminals 5a are directly formed on the opposing surface of the substrate 2, the terminals 5a of the electrodes 5 will not separate from the substrate 2 because the material for the electrodes 5, such as ITO, has a good adherence to the substrate 2 made of glass.
On the opposing surface of the other substrate 1 (the upper substrate in FIG. 1) a plurality of striped display electrodes (electrodes made of a transparent material such as ITO) 6 are formed, and the extending direction of the striped display electrodes 6 are perpendicular to that of the striped display electrodes 5 on the above described one substrate 2. The above described one striped electrodes 5 are used as scanning electrodes (common electrodes) and the other striped electrodes 6 are used as signal electrodes (segment electrodes). Both the striped electrodes 6 on the opposing surface of the upper substrate 1 and the striped electrodes 5 on the opposing surface of the lower substrate 2 are respectively covered by aligning films 7, 7 formed on the opposing surfaces. The substrates 1 and 2 are indirectly bonded through a frame-shaped sealing member 8 interposed between the peripheral areas of the opposing surfaces thereof, and a space surrounded by the sealing member 8 between the substrates 1, 2 is filled with liquid crystal material 9.
The protective film 4 is firstly formed as a whole surface covering film 4a covering the whole opposing surface of the substrate, as shown in FIG. 2A, by applying a transparent resin material such as polyimide through a spin-coat method to the whole opposing surface of the substrate 2, on which the color filters 3R, 3G and 3B are formed, to make a thick film layer and then baking the thick film layer. The whole surface covering film 4a is etched at its peripheral area to be removed away to have a predetermined configuration for the protective film 4, as illustrated in FIG. 2B.
Alternatively, the protective film 4 can be formed by a technique similar to offset print. With this technique, a projecting pattern identical with that of the protective film is formed on the peripheral surface of a printing drum. Then, after the material of the protective film is evenly applied on the projecting pattern, the drum is rotated with the projecting pattern being pressed on the opposing surface of the substrate, so that the material of the protective film, having the same shape as that of the projecting pattern, is applied on the opposing surface of the substrate. The applied film is predried and then heated to be hardened to the form a protective film.
FIG. 3 shows a liquid crystal display device having a protective film formed on a substrate 20 by the above described technique similar to offset print. In FIG. 3, on an opposing surface of the lower substrate 20 color filters 30 including red, green and blue ones are formed, and further a protective film 40 is formed thereon to cover the color filters 30. Display electrodes 50 are formed on the protective film 40, and an aligning film 70 is further formed on the electrodes 50. On the opposing surface of the upper substrate 10 a plurality of striped display electrodes 60 are formed, and on the electrodes 60 an aligning film 70 is formed. The lower substrate 20 and the upper substrate 10 are bonded through a sealing member 80 interposed therebetween, and a space surrounded by the sealing member 80 between the upper and lower substrates 10 and 20 is filled with a liquid crystal material 90.
The two conventional liquid crystal display devices as illustrated in FIGS. 1 to 2B, and 3 has a drawback that the thickness of the protective film 4 or 40 at the edge portion of the color filter 3 or 30 is thin, because the material of the protective film still having a fluidity flows down from higher to lower regions on the lower substrate 2 or 20 while it is in the preliminary drying process after it has been applied onto the color filters 3 or 30 on the lower substrate 2 or 20 by means of the spin-coat technique or printing technique as described above.
Particularly, in a liquid crystal color display device having color filters 31 various color filters of which have different thickness, the liquid crystal material changing its thickness at its various points, as illustrated in FIGS. 4A and 4B, in order to control an intensity of transmitted light passing through the color filters and hence improve the color balance in the whole area of the filter 31, the material 41a of the protective film which has not hardened yet flows down from higher to lower regions on the lower substrate 21 so that the thickness of the material 41a applied on the substrate 21 becomes thin at the edge portions of the color filters 31R, 31G and 31B formed on the lower substrate 21.
A protective film material having high viscosity may be used in order to eliminate the above described drawback, but such high viscous material injures the flatness of the surface of protective film as described later.
Moreover, in the conventional liquid crystal display devices as described above, the outer peripheral end surface of the protective film 4 or 40 formed on the substrate 2 or 20 to cover the color filters 3 or 30 is a steep slope, and the protective film 4 or 40 has an enough thickness to effectively cover the color filters 3 or 30 formed on the opposing surface of the substrate 2 or 20. Therefore, the terminals of the display electrodes 5 or 50 formed to extend from the upper surface of the protective film 4 or 40 to the outer peripheral portion of the opposing surface of the substrate 2 or 20 are tend to be broken at the steeply inclined outer peripheral end surface.
The reason why the terminals are tend to be broken is because the electrodes 5 or 50 are formed of a transparent conductive material such as ITO by means of spattering technique. Since such a conductive material for the electrodes 5 or 50 can hardly be deposited by the spattering technique on the outer peripheral end surface of the protective film 4 or 40 which is the steep slope and has a big height, the thickness of the conductive material is extremely reduced at the outer peripheral end surface. Therefore, when electrodes 5 or 50 are formed by patterning the above described conductive layer by means of photolithography technique, thin height portions of the transparent electrodes 5 or 50 which are arranged at the outer peripheral end surface of the protective film 4 or 40 to form terminal members are broken.
Besides, in the above described conventional liquid crystal display device, since the lower substrate 2 or 20, the color filters 3 or 30, the protective film 4 or 40 and the display electrodes 5 or 50 formed on the protective film have different thermal expansion coefficients, the stress, generated within the protective film 4 or 40 due to the difference of thermal expansion coefficients during a heat treatment process after the formation of the display electrodes 5 or 50, will concentrate in the thin thickness portion of the protective film and produce cracks in that portion. Such cracks are one of causes to break the display electrodes 5 or 50 formed on the protective film 4 or 40.
Furthermore, if a printing technique as shown in FIG. 3 is used to form a protective film by using a highly viscous protective film material, undulation 42b is formed on the surface of the whole surface protective film 42a printed on the lower substrate 22 to cover its entire opposing surface, as illustrated in FIGS. 5A and 5B. The undulation 42b remains after the whole surface protective film 42a is preliminary heated and then shaped as the protective film 42, so that the undulation 42b causes the thickness of the liquid crystal layer to be uneven in the finally formed liquid crystal display device. The uneven thickness of the liquid crystal layer causes the intensity of the transmitted light to be varied, so that the undulation 42b is appeared on the display of the display device.
In a case that the protective film material is applied on the opposing surface of the substrate by the above described printing technique, if a printing drum 43 having a projecting pattern as shown in FIG. 6A is used, a wall 42c is formed on the outer peripheral portion of the whole surface protective film 42a (the peripheral area of the end surface of the projecting pattern of the printing drum 43), as shown in FIG. 6B. The height of the wall 42c is approximately proportional to the thickness of the whole surface protective film 42a, so that as the thickness of the film is thicker as the height of the wall 42c is greater.